Search portals like Google® and Yahoo® are among the top daily visited websites, indicating how crucial these services are to end users. In response to their popularity, search engine providers have put tremendous effort in optimizing their infrastructure, usually either through server-side or client-side caching, to enable the fastest possible search user experience.
Unfortunately, these optimizations can be rendered ineffective or costly due to the unpredictable nature of the search query volume. In particular, important events that take place in the physical world can instantly translate into significant, and often unpredictable, spikes in search requests. For instance, right after terrorist attacks, such as the 9/11 or more recently the Boston marathon bombing, users tend to search the web to get details about the events. Similarly, as other major ‘trending’ events, such as elections, are taking place, large numbers of users simultaneously search the web to get informed about the results. These search volume spikes, even though not happening daily, have significant impact on both the search backend and the mobile user experience.
First, datacenters are overprovisioned to sustain worst-case scenarios such as search volume spikes. The larger the search volume spikes, the higher the cost of overprovisioning (more active servers in the data center) or the higher the delay that end users experience. Server-side caching techniques can reduce the impact that each of these trending requests has on the datacenter, but they cannot prevent large numbers of requests from hitting the datacenter's front end. Second, in the presence of such unpredictable events, end users lose the benefit of real-time search experience delivered by previously proposed approaches for client-side search index caching. This negative impact is particularly crucial to mobile devices, equipped with energy consuming cellular links that are limited by lengthy connection setup times. As these devices are quickly settling to become the major entry point for accessing search services, the mobile experience delivered becomes a driving optimization factor for search engines.